Polyethylene terephthalate (hereinafter, sometimes abbreviated as “PET”.) is a polyester resin widely used for sheets, films, containers and the like because of having characteristics such as excellent transparency, mechanical strength, chemical resistance and recyclability.
In particular, a PET transparent molded article produced by injection molding is highly in demand in applications such as sundries and containers because of being excellent in functionalities such as lightness and impact resistance and also being large in the degree of freedom of shape design.
In addition, a PET transparent sheet is increased in demand in the food field because of having the advantages of being excellent in environmental compatibility, for example, generating no dioxin in incineration, being recyclable, and generating no environmental hormone.
Furthermore, a PET transparent container is increased in demand in the food field and the toiletry field because of having the advantages of being excellent in functionalities such as lightness, ease of handling and identifiability of contents, and in environmental compatibility, for example, generating no dioxin in incineration and being recyclable.
PET, however, has a glass transition temperature of about 80° C., and therefore is not sufficiently high in heat resistance and cannot be utilized as a material that can be applied to high-temperature sterilization like boiling disinfection, cooking in a microwave oven, and the like. In addition, for example, when a thick molded article is obtained and when a sheet is subjected to secondary molding to provide a molded article, high crystallinity may cause whitening to progress, thereby impairing transparency. Therefore, modification by copolymerization has been widely performed.
As an example of the modification, an amorphous polyester resin has been proposed in which PET is copolymerized with isophthalic acid or copolymerized with 1,4-cyclohexane dimethanol to result in a reduction in crystallinity, thereby reducing the problem of whitening of a molded article. Such a resin, however, has a glass transition temperature of below 100° C., and is not almost improved in terms of heat resistance.
Polyethylene naphtha late (hereinafter, sometimes abbreviated as “PEN”.) has been utilized in the above application where heat resistance is required, but is equivalent to PET in crystallinity, and therefore may be impaired in terms of transparency in the case of being formed into a thick molded article.
As another example of the modification by copolymerization, Patent Documents 1 and 2 disclose a polyester resin obtained by copolymerization of a diol having a pentacyclopentadecane backbone. In particular, Patent Document 1 describes a polyester resin in which a dicarboxylic acid unit is mainly a unit derived from an aliphatic dicarboxylic acid and a diol unit other than the diol having a pentacyclopentadecane backbone is a unit derived from an aliphatic diol. In addition, Patent Document 2 describes a polyester resin in which an aromatic dicarboxylic acid unit or an aliphatic dicarboxylic acid unit as a dicarboxylic acid unit, and a diol having a pentacyclopentadecane backbone as a diol unit are copolymerized.